Lets be scientific about this.
The experimental evidence is this:
People report having seen images/shadows or heard noises relating to people or objects that either could not by probable convention have been there or disappeared in a manor not associated with such people or objects.
Possible explanations:
- The person seen/heard really was there however unlikely that might have been perceived to be.
Supporting evidence: People are sometimes in your garden when they shouldn't be. Or even walking in your bedroom. Sometimes they leave surprisingly quickly while you were distracted. Images that look like people can be generated by random reflection, diffraction or mirage. The human brain is trained to see people even when they aren't there as any number of pictures of random noise in which people can 'see' faces will attest too.
- People are lying.
Supporting evidence: People lie about all sorts of things all the time for all sorts of reasons. There is no particular reason to discount lying as an explanation for a ghost story unless you were the person who experienced it.
- The phenomenon they heard or saw was not objectively real but experienced due to the complexity of the human brain and senses.
Supporting evidence: The human brain is very complex and poorly understood. Hallucinations happen. Dreaming happens. Disorders in which people become unable to tell real from imagined events exist. Memory is massively susceptible both to loss of information and to rewriting by suggestion repeated rehashing or by the normal processes used by your brain to compensate for varying speeds of information flow (yep everyone's brain lies to them constantly about what they are sensing in order to back calculate when something most likely happened even though the information arrived later...). A screw up by any of these systems as a one off or as part of a known but undiagnosed brain condition could easily explain any ghost experience.
- Time slips.
Supporting evidence: None that I am aware of. As far as I know there is no data to suggest such a thing has ever happened.
- Ghosts exist.
Supporting evidence: Again, none that I am aware of. We have never been able to convincing rule out all the existing possibilities in order to leave this as the only remaining one.
To be honest the big problem with ghost hunting is number 3 on this list. Until someone sees an impossible person while having their brain scanned, said scan indicating their visual systems operating normally and so actually seeing what cannot be there, we aren't going to be able to rule out brain fart as an explanation for seeing a ghost.